/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Append triangles of an existing mesh. This duplicates the current groups and also any attributes existing on the triangles.
void AppendTriangles ( const [FDynamicMesh3](API\Runtime\GeometryCore\DynamicMesh\FDynamicMesh3) * SourceMesh, const [TArrayView](API\Runtime\Core\Containers\TArrayView)< const int > & SourceTriangles, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) & ResultOut, bool bComputeTriangleMap )